The goal of the MOOD-PD Study is to optimize detection of depressive disorders in patients with Parkinson's disease (PD). PD affects 1% of the population over age 65. While progressive motor abnormalities are the hallmark of PD, depression affects up to 50% of patients, contributes to impaired quality of life more than motor dysfunction, aggravates motor and cognitive deficits, and is a major source of excess disability, economic strain, and caregiver distress. Yet, clinicians often fail to diagnose or treat depression in PD adequately. Since most PD patients do not see psychiatrists, specialists in both PD and mood disorders have identified the need for valid methods to facilitate depression recognition by nonpsychiatric clinicians. To address under-recognition of depression in the medically ill, the US Preventive Services Task Force now recommends routine depression screening in primary care settings. However, generic approaches cannot be recommended for PD patients without specific assessment of their validity. This is because clinical features of depression overlap directly with those of PD itself, most depressive disorders in PD do not meet criteria for major depression, and other common psychiatric co-morbidities in PD (e.g., apathy and anxiety) can be misdiagnosed as depression. In addition, time constraints limit comprehensive assessments of motor and mood status. Thus, optimal detection of depression in PD requires valid and efficient screening methods. In order to provide such methods, the MOOD-PD study will draw on a sample of 800 PD patients from two community-based neurology practices. Using a two-stage design for screening and diagnosis, the psychometric properties of depression rating instruments will be assessed relative to the best estimate diagnoses determined by a panel of psychiatrists with expertise in geriatric psychiatry and movement disorders. Unlike previous studies, results of the MOOD-PD study will be directly applicable in clinical settings. The Specific Aims are: 1) To evaluate and compare the psychometric properties of existing depression assessment tools, relative to gold-standard diagnostic procedures, for identifying depressive disorders (e.g., major depression and minor depression) in PD patients;2) To determine the influence of other nondepressive affective syndromes (e.g., anxiety and apathy) on performance of depression rating tools for detecting depressive disorders in PD patients;3) To conduct follow-up assessments in the patients with the diagnosis of depression, as determined by the gold-standard procedures, to evaluate the ability of the depression measures to identify persistent depression;4)To develop guidelines using depression assessment tools for optimizing the detection of clinically significant depression in PD, based on the results of Aims 1-3.